Apparatus for firing ceramic sparkplug insulators

ABSTRACT

Apparatus for firing sparkplug insulators comprises a jig having an apertured bottom wall. A pin having a free end and a collar situated adjacent the latter is arranged to move vertically from below the bottom wall through a respective aperture therein. A sleeve slidably mounted on the pin is introduced into the aperture to be expanded therein in response to a further upward movement of the pin beyond the point at which a stop prevents the sleeve from moving upwardly, and thus to center the pin in respect to the aperture. The free end of the pin engages the central hole of an insulator, the extent of penetration into the latter being limited by the collar and the pin with the thus centered insulator is then moved downwardly so as to introduce the insulator into the aperture, while the collar prevents the sleeve from enclosing the free end of the pin.

United States Patent 119 Duffner Jan. 8, 1974 APPARATUS FOR FIRING CERAMIC Primary Examiner-Thomas H. Eager SPARKPLUG INSULATORS Att0rneyMichael S. Striker [75] Inventor: Josef Duffner, Bamberg, Germany [57] ABSTRACT [73] Ass1gnee: Robert Bosch GmbH, Stuttgart,

Germany Apparatus for-firing sparkplug msulators compr ses a ig havmg an apertured bottom wall. A pm having a Filedi 0, 1972 free end and a collar situated adjacent the latter is arranged to move vertically from below the bottom wall [2]] Appl 279634 through a respective aperture therein. A sleeve slidl ably mounted on the pin is introduced into the aper- Foreign Appli a ion Priority Data ture to be expanded therein in response to a further Sept. 9, 1971 Germany ..1 2145 057.7 upward movement of the P beyond the point at which a stop prevents the sleeve from moving up- [52] us. Cl. 29/203 P a y, and thus to center the p n in sp to t p- [51] Int. Cl. H05k 13/04 erture: The r n of h pin ngage he n ral [58] Field of Search 29/203 P, 203 R, hole of an insulator, h extent of penetration into t 29/200 P, 203 J, 200 J latter being limited by the collar and the pin with the t thus centered insulator is then moved downwardly so [56] R fe c Cit d as to introduce the insulator into the aperture, while UNITED STATES PATENTS the collar prevents the sleeve from enclosing the free 3,667,103 6/1972 Petree 29 203 v of the "11 Claims, 5 Drawing Figures PATENTEDJAN 8 I924 SHEEI 10F 3 1. l EU Mm PATENTEDJAH 8 1374 saw an; a

Fig. 3

APPARATUS FOR FIRING CERAMIC SPARKPLUG INSULATORS BACKGROUND OF THE INVENTION The present invention relates to an apparatus for firing ceramic sparkplug insulators, comprising a jig and apparatus for automatically loading and unloading the same.

Sparkplug insulators are conventionally made of a suitable ceramic material, or a combination of such materials, and are shaped to the desired configuration. However, when they are so shaped from such material, the insulators are soft and can be given the necessary high mechanical strength and electrical insulation characteristics only by being fired in a firing oven, usually at temperatures of between substantially 1,500 and 1,650 C. When these blanks, that is the sparkplug insulators which have been formed from ceramic material but. have not yet been fired, are to be introduced into the firing oven, they are conventionally inserted into a jig provided for this purpose, usually called a burner capsule. Such jigs are as a rule configurated as containers of rectangular or quadrangular outline and are themselves bodies consisting of fire-proof ceramic material.

The jigs must of course have sufficient mechanical strength, in order to be capable of supporting and retaining the ceramic sparkplug insulators even at the highest firing temperatures which are to be encountered. To assure this, it was heretofore thought that particularly the bottom wall of the jig must be very thick. This, of course, results in a significant reduction of the usable proportion of the interior space of the firing oven, especially in view of the fact that it is conventional to stack six of the jigs above oneanother so that in height six times the thickness of the bottom wall of the respective jig is lost as unusable space.

Evidently, this is a disadvantage attendant to the prior art. An additional disadvantage is to be found in the fact that these known jigs cannot be produced according to precise dimensions. They shrink by substantially 1-2 percent during their own firing, that is when they themselves are fired to impart to them the necessary mechanical strength and during further firings -during which they support ceramic sparkplug insulatorsthey may undergo other dimensional changes, for instance they may also increase in size. As a result of all this, it has been found that it is to all intents and purposes impossible to producea jig in which the sparkplug insulators will always be in contact with one another and with the walls bounding the jig, without any freedom of play. Another reason why this is impossible resides in the fact that the soft un-fired sparkplug insulator blanks also differ from one another in their diameters.

For the above dimensional reasons, it has heretofore beennecessary to tightly pack or position sparkplug insulatorsto be fired in the jigs of the prior art, carrying this out by hand and filling any remaining gaps between adjacent insulators with ceramic filler elements and wedges, a procedure which is necessary to assure that during the transportation of the jig into the firing oven the not-yet-fired sparkplug insulators cannot move relative to one another and thus become damaged.

From what has been set forth above, it is clear that a significant disadvantage of the prior art resides in the fact that the utilization of the prior-art jigs is time consuming, thus increasing the expenses involved in producing the sparkplug insulators and, ultimately, in producingthe sparkplug itself.

It is therefore evident that it would be desirable to provide an apparatus which would automatically insert the sparkplug insulators into the jig so as to be properly positioned in respect to one another and secured to the jig so as to prevent the insulators from being damaged during the transportation of the jig and further to utilize the idle space taken by the relatively thick bottom of the jig for accommodation of at least a portion of each of the insulators. However, no such apparatus and asso ciated jig is known from the prior art. I

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to overcome'the disadvantages of the prior art.

More particularly, it is an object of the present invention to produce an improved jig for firing of ceramic sparkplug'insulators, which is simple in construction and economical to use.

Still another object of the present invention is to provide such an improved jig which makes it possible to insert the sparkplug insulatorsinto the jig mechanically and automatically, rather than manually.

A concomitant object of the present invention is to providesuch an improved jig whose bottom is provided with a plurality of apertures each permitting insertion of an inserting apparatus into the jig so as to engage a respective sparkplug insulator and locate the same in the aperture so that a portion thereof is accommodated in the aperture in the bottom wall.

A further object of the present invention is to provide a jig which reliably prevents damage of the inserted sparkplug insulators.

A still further object of the present invention is to provide an apparatus, which is simple in construction, for automatically inserting the insulators into the jig.

An additional object of the present invention is to provide such an apparatus for inserting the sparkplug insulators into the apertures of the bottom wall of the jig which reliably centers the sparkplug insulators in respect to the respective apertures during their insertion.

According to the present invention, an apparatus for firing stepped-diameter ceramic sparkplug insulators each having a larger diameter section and a smaller diameter section, comprises a jig having an apertured bottom wall. Each aperture is adapted to receive and retain the smaller diameter section of a respective insulator and to prevent the larger diameter section of said respective insulator from entering therein so that the latter may rest on said bottom wall while partially in serted into an aperture. Engaging :means are provided for acquiring and centering the insulators during automatic loading of said jig.

In accordance with a presently preferred embodiment, each aperture in the region where an insulator is to rest on the bottom wall of said jig has a diameter greater than that of the smaller diameter section of the insulator prior to firing, and smaller than that of the larger diameter section of the insulator subsequent to firing. With insulators having a central bore, said bottom wall is normally arranged horizontally while loading of the jig with insulators is carried out by lowering the latter into the jig from above. Said engaging means comprises pin means having a free end dimensioned to matingly engage the central bores of the insulators.

Said pin means is arranged to move vertically along the axis of an aperture being filled between a first position wherein said free end is below said bottom wall, and a second position in which said free end has penetrated a respective aperture and projects upwardly above said bottom wall. Clamping means cooperative with said pin means and said apertures to maintain said pin means coaxially aligned with the axes of the respective apertures in which said pin means is positioned.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a partially sectioned perspective view illustrating a jig according to the present invention, with two sparkplug insulators inserted therein;

FIG. 2 is a sectional elevation of the jig shown in FIG. 1 with sparkplug insulators inserted therein, and further illustrating means for inserting the sparkplug insulators into the jig prior to firing; 7

FIG. 3 is a sectional elevation of the jig illustrated in FIG. 1 with sparkplug insulators inserted therein, and further illustrating means for removing the sparkplug insulators subsequent to firing; and

FIGS. 4 and 5 are sectional elevational views showing two further embodiments of the jig of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing the drawing in detail, and firstly the embodiment illustrated in FIG. 1, it will be seen that the jig in accordance with the present invention is in the form of an essentially cup-shaped element of quadratic or rectrangular configuration. However, the specific configuration is not critical. The bottom wall 10 may have a thickness of.approximately 35 millimeters and is provided with a plurality of apertures 11 which, in the normally horizontal position of the bottom wall 10, have their axes extend substantially vertically.

The sparkplug insulators 25 which are to be fired, each essentially comprise a larger diameter section 12 and a smaller diameter section 13 the diameter of the smaller section 13 being somewhat smaller than that of the apertures 11 so that the sections may be fitted therein with some clearance. During firing, the insulators 25 rest on the top surface 10a, as seen in FIG. 1, of the bottom wall 10' since the diameters of the larger sections 12 are greater than the diameters of the apertures 11 which latter are bounded by upper rims 11a. Preferably, the diameter of the apertures 11 should be selected so that it is larger than that of the smaller sections 13 prior to firing and smaller than that of the larger sections 12 subsequent to firing. Two side walls 14 of the jig 10 project from the bottom wall 10' as shown.

The distances between the centers of adjacent ones of the apertures 11 are so selected that they are only slightly greater than the diameter of the insulator sections 12, in order to assure maximum utilization of the surface area of the bottom wall 10. This means that prior to filing the sections 12 of the various adjacent insulator blanks will either contact one another, or that there will be very small play between them when an insulator blank is inserted into each of the apertures of the jig.

The jig according to the present invention is advantageously loaded with insulator blanks not manually, but automatically. For this purpose, the jig is placed onto a suitable conveyor which moves past a device which sequentially inserts blanks into the various apertures of the jig. This device is illustrated in FIG. 2 and comprises a guide pin 15, whose free end 16 is so dimensioned that it can matingly engage a central bore 17 of a plug insulator 25. The guide pin 15 is provided with a plurality of stepped sections, such as stop means 16a which is in the form of an enlarged diameter collar and serves for supporting the sparkplug insulator 25 and for preventing the free end 16 from penetrating into the bore 17 to an excessive depth.

On the guide pin 15 there is provided an axially slidable expandable sleeve 18 which has outer and inner cylindrical surfaces. The sleeve 18 has a stop portion 18', at the inner cylindrical surface of which there is provided a tapered portion 18a which defines therein a space whose cross section gradually decreases in the upward direction. Positioned inside the sleeve 18 is a bearing means in the form of an annular tapered bushing 19. The bushing 19 is normally positioned near the region of the top portion 18 to cooperate with the portion 18a, as will hereafter be described. The pin 15 is also provided with a stepped portion 15 which serves as a support for a spring 20 which acts upon the bushing 19 and urges the same in an upward direction relative to the position of the guide pin 15. The arrow 19' illustrates the direction of the force applied by the spring 20 upon the bushing 19.

The sleeve 18, as suggested above, is expandable. In accordance with the present invention, the sleeve 18 is preferably expandable in radial direction in the region of the top portion 18'. For this purpose, the sleeve 18 may consist of two somewhat flexible semicylindrical shell sections. Another possibility is a sleeve which is longitudinally slotted at least in the region of the top portion 18. By forcing the bushing 19 in an upward direction to thereby cause the latter to act on the tapered protion 180, the top portion 18 of the sleeve 18 expands in the radial direction of the axis of the guide pin 15. It should be noted that the normally unstressed sleeve 18 has an effective diameter which is slightly smaller than that of the apertures 11, so as to leave some clearance and facilitate insertion of the sleeve into a respective aperture. Now, when the bushing 19 acts upon the tapered portion 18a, the top portion 18' expands as described above, and the effective diameter of the upper portion 18' becomes equal to that of the respective aperture 11 to thereby result in engagement of the outer surface of the top portion 18' with the inner surface of the aperture 11. By this expedient, it is assured that the sleeve 18 is centered coaxially within the aperture 11 and, since the bushing 19 engages both the tapered portion 18a as well as the guide pin 15, the latter will similarly be axially centered. The sleeve 18 may be thought of as a tong vise which holds the guide pin 15 aligned with the axis of the sleeve 18 prior to engagement of an insulator blank with the aperture, and which maintains the pin 15 aligned with both the sleeve 18 and the aperture 11 until insertion of the blank 25 into the aperture 11 has been effected.

The pin 15 is arranged for movement in an upward and downward direction, as illustrated by the arrow 15a; by means of a spring 21 a shoulder 22 of the pin 15 acts 'on a flange 23 of the sleeve 18. The flange 23 is provided with a bearing surface 23. The spring 21 urges the sleeve 18 in an upward direction, as viewed in FIG. 2, the movement of the sleeve 18 being limited by a stop 24 which is arranged to engage the bearing surface 23.

The operation of the apparatus will now be described:

The jig is placed on a carrier and brought to a position below a supplying or feeding apparatus which is located above the jig. The pin 15, initially in its lowered position, is aligned with one of the apertures 11 and is moved upwardly so that its free end 16 penetrates through the bottom wall 10' and extends towards the supplying apparatus. As the pin 15 moves upwardly,

the sleeve 18 similarly moves upwardly by the action of the spring 21. Prior to the time that the stop portion 18' has fully penetrated the bottom wall 10', the relative axial positions of the pin 15 and the sleeve 18 are such that the bushing 19 does not yet engage the tapered portion 18a and therefore the effective diameter of the top portion 18 is less than that of the aperture 11. The initially smaller diameter of the sleeve 18 relative to the aperture 11 facilitates the insertion of the sleeve 18 into the latter. Once the top portion 18' has at least partially penetrated the bottom wall 10, the stop 24 is effective for preventing further movement of the sleeve 18 in the upward direction. With further upward movement of the pin 15, the bushing 19 is brought to bear against the tapered portion 18a by means of the spring 20. At this time, as described above, the diameter of the upper portion or top portion 18 expands until it equals that of the respective aperture 11 so that no clearance or play is provided between the two cylindrical surfaces this assuring that the sleeve 18 as well as the pin 15 are perfectly aligned with the axis of the aperture 11. The pin 15 continues to'move upwardly, all

the time being centered by the bushing 19 and the top portion 18', until the free end 16 enters the central bore 17 of the sparkplug insulator 25. At this time, the feeding apparatus releases the sparkplug insulator 25 so that the latter is brought to rest on the stop means or collar [60 as shown in FIG. 2. Now, the pin 15 begins to move downwardly. However, because of the compressed state of the spring 20, the bushing 19 remains engaged with the tapered portion 18a so that the pin 15 remains coaxially aligned with the axis of the aperture 11 until when the top portion 18' is only partially inserted into the aperture 11 at least a portion of the smaller diameter section 13 of the insulator 25 is inserted into the aperture 11. After the smaller diameter section 13 has been inserted into the aperture 11, further downward movement of the pin 15 allows the spring to relax until it reaches its uncompressed state and until the bushing 19 is no longer forced against the tapered portion 18a by the spring 20. At this time, as described above, the sections of the sleeve 18 in the region of the top portion 18 will return to their original unstressed position so that the effective diameter of the top portion 18 again becomes less than that of the aperture 11. At this time, also the entire sleeve 18 as well as the pin 15 may be freely removed from the aperture 11 and moved into registry with another aper ture for similar filling.

The lower portion of the guide pin 15 can be connected to a pneumatic, hydraulic or mechanical apparatus not shown)'which moves the guide pin 15 up wardly and downwardly in a programmed fashion so that the above process may be repeated successively with a plurality of apertures 11. However, the driving mechanism for moving the pin 15 is not a part of the present invention except that it is point out that utilization of such a device is possible in order to fully automate the loading procedure as contemplated by the present invention. Similarly, the feeding apparatus referred to above is not shown an automatic gripping device of the type contemplated for use with the present invention being fully known and not forming part of the present invention. By utilizing the present invention, it is possible for the sparkplug insulator to be precisely centered prior to insertion into the apertures of the jig. The above described apparatus is advantageously used either singly as shown in FIG. 2 to fill one aperture at a time or a plurality of such apparatus may be utilized in rows for successively filling rows of apertures until the entire jig is loaded. A particular advan- V tage of the present invention in connection with the apparatus above described consists in that the loading of a jig utilized in firing such insulators becomes less dependent on the precise diameters of the holes in the insulators. More particularly, the utilization of such a jig no longer depends on the diameter of the holes as critically as in the prior art and is simiiarly not so critically dependent on the changes in the diameters of the holes of the jig materials and therewith also on the distances between the apertures the sparkplug insulators necessarily being centered in each instance by the abovedescribed device.

Of course, the novel jig will undergo dimensional changes during repeated firing, as do the jigs known from the prior art. As a result of this the spacing between the apertures and the jig will also change over a period of time and the differences between the always identical spacing between adjacent inserting stations of the automatic inserting device and the changes in the dimensions between the adjacentapertures in the jig will be cumulative over a period of time so that, when dimensional changes in the jig have occured, in subsequent filling operations the apertures of the jig will no longer be properly aligned with the filling stations of the automatic filling device during; the entire sequence of filling operations. This means that proper insertion of the insulator blank at each filling station into a re spective aperture of the jig would no longer be assured. This, however, can be avoided by providing the abovedescribed guide pin.

It should not be assumed, however, that the apertures l 1 are provided only in order to permit the insertion of the aforementioned pin. Rather, they have other purposes also, for instance, during the firing process every plug 25 will contract in its diameter and length by approximately 20 percent, so that it has a rather large play in the apertures. In order to assure that no damage occurs to the converted blanks, that is to the fired insulators, which still are susceptible to such damage even after firing, pins 30 as illustrated in FIG. 3 are inserted from below into the respective apertures l 1 after firing, lifting the fired insulators upwardly until they can be gripped by suitable gripping devices (not shown) at the junction between their large and smaller sections 12 and 13, and can be removed automatically to be deposited on repositories provided for this purpose.

It will be appreciated that the novel apparatus thus permits not only the automatic loading but also the automatic unloading of the jig, thereby increasing substantially the speed of loading and unloading and decreasing substantially the expenses involved in these operations.

Moreover, the provision of the apertures in the bottom wall 14 of the novel jig increases the capacity of the jig for handling the insulators. The conventional jigs, in which the bottom wall is not apertured and the insulators rest on the top surface of the bottom wall, are known to have bottom wall thicknesses of approximately 22 millimeters. This corresponds to a loss of space in the firing oven of approximately the same dimension per jig, i.e., the overall height of the loaded jig is the sum of the thickness of the bottom wall and of the length of the insulators which rest in upright position on the upper surface of the same. However, with the novel jig according to the present invention, the insulators are over a significant part of their length accommodated in the apertures of the bottom wall so that the lost space above the upper surface of the bottom wall need only be e.g., 6mm, the distance by which the insulators project upwardly of the upper surface. This represents a saving of 226=l 6 millimeters in the oven space per firing jig. lf six of these jigs are stacked above one another in the firing chamber of an oven, a total of 96 millimeters can be saved. This dimension corresponds approximately to the height of one of the jigs, so that it is now possible to stack seven jigs in the space which previously was required for only six jigs, without having to change the dimensions of the oven chamber. This means an increase of 16.7 percent in the number of sparkplug insulators which can be fired at one time. Furthermore, due to the fact that the jig according to the present invention need have only two side walls 14 instead of four side walls required in the prior art'construction (where the insulators were not retained in apertures of the bottom wall), a further increase in the ability, of the jig to hold sparkplug insulators is achieved, amounting to approximately 7 percent because the space previously required for the two now omitted side walls can be used for apertures in which sparkplug insulators can be accommodated.

Referring to FIGS. 4 and 5, these show the jigs 10 having apertures 11' and 11'', respectively, which are tapered in a sense first increasing downwardly and then increasing upwardly in their respective diameters. in

both cases, it is only importantthat the guide pin 15 be centered prior to the point when a portion of the smaller diameter insulator section 13 has moved below the rims 11a.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of firing apparatus differing from the types described above.

While the invention has been illustrated and described as embodied in apparatus for loading a jig for firing of ceramic sparkplug insulators, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. Apparatus for firing stepped-diameter ceramic spark plug insulators each having a larger diameter section and a smaller diameter section, comprising a jig having a bottom wall provided with a plurality of apertures each adapted to receive and retain the smaller diameter section of a respective insulator to prevent the larger diameter section of said respective insulator from entering therein so that the insulator may rest on said bottom wall while partially inserted into the respective aperture; and engaging means for acquiring and centering a respective insulator during automatic loading of said jig.

2. Apparatus as defined in claim 1, wherein said apertures are tapered.

3. Apparatus as defined in claim 1, wherein the diameter of each aperture in the region where an insulator is to rest on said jig is greater than that of the smaller diameter section of the insulator prior to firing and smaller than that of the larger diameter section of the insulator subsequent to firing.

4. Apparatus as defined in claim 1, wherein said bottom all has a top surface on which the larger diameter sections rest, and wherein each aperture is banded at said top surface by a rim which acts as a centering means for the positioning of the insulators in said jig during the loading of the latter.

5. Apparatus as defined in claim 1, each insulator having a central bore, and said bottom wall being normally arranged horizontally while loading said jig with insulators is effected by lowering the latter into the apertures of the jig from above; and wherein said engaging means comprises pin means having a free end dimensioned to matingly enter into the central bores of the insulators, said pin means being arranged to move vertically along the axis of an aperture being filled between a first position wherein said free end is below said bottom wall and a second position wherein said free end has penetrated a respective aperture and extends above said bottom wall; and clamping means cooperating with said pin means and the respective aperture for maintaining said pin means coaxially aligned with the axis of the respective aperture while said pin means are positioned therein.

6. Apparatus as defined in claim 5, wherein said free end is provided with stop means for limiting the extent of penetration of said free end into the central bore of an insulator.

7. Apparatus as defined in claim 6, wherein said clamping means comprises expandable sleeve means coaxial with said pin means and enclosing at least a portion of said pin means with freedom of axial movement along the latter, said stop means being operative to limit the movement of said sleeve means above said stop means in a sense enclosing said free end.

8. Apparatus as defined in claim 6, wherein said sleeve means has inner and outer cylindrical surfaces and a top portion which is expandable in radial direction whereby said outer cylindrical surface only frictionally engages said apertures when said top portion is expanded, said inner surface of said top portion including a tapered section; and further comprising bearing means cooperating with said pin means and said tapered section for expanding said sleeve means in response to upward movement of said pin means relative to said sleeve means beyond a predetermined extent while the insulator is outside of a respective aperture, to thereby maintain coaxial alignment of said pin means at least until the insulator is inserted into said aperture.

9. Apparatus as defined in claim 8, wherein said bearing means comprises an annular tapered bushing coaxially mounted for slidable movement on said pin means; and further comprising biasing means cooperating with said pin means to urge said bushing against said tapered section to thereby expand said sleeve means.

10. Apparatus as defined in claim 8, wherein said sleeve means extends axially below said bottom wall and is provided with a bearing surface below said bottom wall, and further comprising another stop means for engaging said' bearing surface to limit the upward travel of said sleeve means to thereby permit engagement of said bearing means and said tapered section.

11. Apparatus as defined in claim 10, further comprising biassing means cooperating with said pin means and said sleeve means for urging the latter in an upward direction relative to said pin means. 

1. Apparatus for firing stepped-diameter ceramic spark plug insulators each having a larger diameter section and a smaller diameter section, comprising a jig having a bottom wall provided with a plurality of apertures each adapted to receive and retain the smaller diameter section of a respective insulator to prevent the larger diameter section of said respective insulator from entering therein so that the insulator may rest on said bottom wall while partially inserted into the respective aperture; and engaging means for acquiring and centering a respective insulator during automatic loading of said jig.
 2. Apparatus as defined in claim 1, wherein said apertures are tapered.
 3. Apparatus as defined in claim 1, wherein the diameter of each aperture in the region where an insulator is to rest on said jig is greater than that of the smaller diameter section of the insulator prior to firing and smaller than that of the larger diameter section of the insulator subsequent to firing.
 4. Apparatus as defined in claim 1, wherein said bottom all has a top surface on which the larger diameter sections rest, and wherein each aperture is banded at said top surface by a rim which acts as a centering means for the positioning of the insulators in said jig during the loading of the latter.
 5. Apparatus as defined in claim 1, each insulator having a central bore, and said bottom wall being normally arranged horizontally while loading said jig with insulators is effected by lowering the latter into the apertures of the jig from above; and wherein said engaging means comprises pin means having a free end dimensioned to mAtingly enter into the central bores of the insulators, said pin means bein arranged to move vertically along the axis of an aperture being filled between a first position wherein said free end is below said bottom wall and a second position wherein said free end has penetrated a respective aperture and extends above said bottom wall; and clamping means cooperating with said pin means and the respective aperture for maintaining said pin means coaxially aligned with the axis of the respective aperture while said pin means are positioned therein.
 6. Apparatus as defined in claim 5, wherein said free end is provided with stop means for limiting the extent of penetration of said free end into the central bore of an insulator.
 7. Apparatus as defined in claim 6, wherein said clamping means comprises expandable sleeve means coaxial with said pin means and enclosing at least a portion of said pin means with freedom of axial movement along the latter, said stop means being operative to limit the movement of said sleeve means above said stop means in a sense enclosing said free end.
 8. Apparatus as defined in claim 6, wherein said sleeve means has inner and outer cylindrical surfaces and a top portion which is expandable in radial direction whereby said outer cylindrical surface only frictionally engages said apertures when said top portion is expanded, said inner surface of said top portion including a tapered section; and further comprising bearing means cooperating with said pin means and said tapered section for expanding said sleeve means in response to upward movement of said pin means relative to said sleeve means beyond a predetermined extent while the insulator is outside of a respective aperture, to thereby maintain coaxial alignment of said pin means at least until the insulator is inserted into said aperture.
 9. Apparatus as defined in claim 8, wherein said bearing means comprises an annular tapered bushing coaxially mounted for slidable movement on said pin means; and further comprising biasing means cooperating with said pin means to urge said bushing against said tapered section to thereby expand said sleeve means.
 10. Apparatus as defined in claim 8, wherein said sleeve means extends axially below said bottom wall and is provided with a bearing surface below said bottom wall, and further comprising another stop means for engaging said bearing surface to limit the upward travel of said sleeve means to thereby permit engagement of said bearing means and said tapered section.
 11. Apparatus as defined in claim 10, further comprising biassing means cooperating with said pin means and said sleeve means for urging the latter in an upward direction relative to said pin means. 